As the size of image sensors and the individual pixels that make up the sensing array become smaller as the result of further integration, it becomes important to capture as much of the incident light onto the sensing array as possible.
Approaches to addressing this issue include having the light sensitive element (such as a photodiode) take up as much as the pixel area as possible. Another approach is to use microlenses over each pixel to focus the incident light onto the photodiode. The microlens approach attempts to capture the light that would be normally incident on portions of the pixel outside of the photodiode region.
Further, in a backside illuminated (BSI) device, a flat mirror material may be used to reflect light back through the photodiode, thereby causing more of the incident light to be converted into an electrical signal.